How to understand "cut clean off" in this sentence
In this paragraph:And since many patients lost their thumbs due to trauma, the tissue that surgeons need to reconnect may not be easy to “hook up,” as Rosen phrases it. In some cases where the thumb was cut clean off, all of the necessary vessels are accessible to the surgeon. Burned, blasted, and crushed hands can make things more complicated.
I am confused about here"where the thumb was cut clean off", I mean cut is a "verb" and "clean off" is a verb phrase", and then why they can just use together directly?

Question

Answer

Its an expression meaning the thumb was "completely removed".

Answer

"Clean off" is actually an adverb phrase here. It describes "cut".

"Cut clean off" means "completely removed", but it also tells us the cut was neat. Several rough attempts to cut something off, like hacking or even tearing, don't classify as "clean off".

The rest of the passage tells us that a "clean" cut is easier to deal with than other injuries to a thumb.

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